Chapter 13 Embedded Systems Characteristics of Embedded Operating Systems Embedded System

Chapter 13 Embedded Systems

• Embedded Systems
• Characteristics of Embedded Operating Systems

Embedded System

• A combination of hardware and software designed to perform a dedicated function
• Often, embedded systems are part of a larger system or product,
• e.g., antilock braking system in a car
• Embedded systems are tightly coupled to their environment  imposes real-time constraints by the need to interact with the environment

Examples of Embedded Devices

Differences from typical computer

• A variety of interfaces
• enable the system to measure, manipulate, and interact with the external environment
• human interface may be as simple as a flashing light or as complicated as real-time robotic vision
• Use of a diagnostic port for diagnosing the system
• Use of special purpose hardware to increase performance or safety
• Fixed-function or application-specific software

Roadmap

• Embedded Systems
• Characteristics of Embedded Operating Systems

• Characteristics of Embedded OS

• Using a general-purpose OS for an embedded system may not be possible
• constraint of memory space
• constraint of power consumption
• real-time requirements
• Special-purpose OS designed for the embedded system environment is commonly used.

Characteristics of Embedded OS

• Real-time operation
• correctness of computation depends, in part, on the time at which result is delivered
• Reactive operation
• needs to consider worst-case conditions in execution in order to respond to external events that do not occur at predictable intervals

Characteristics of Embedded OS

• Configurability
• supports flexible configuration so that only the functionality needed for a specific application and hardware suite is provided
• e.g., allows to select only the necessary OS modules to load
• I/O device flexibility
• handles devices by using special tasks instead of integrating their drives into the OS kernel

• Characteristics of Embedded OS

• Streamlined protection mechanisms
• requires limited protection because tested software can be assumed to be reliable
• e.g., I/O instructions need not be privileged instructions that trap to OS  tasks can directly perform their own I/O
• no use of an OS service call  avoid overhead for saving and restoring the task context

• Characteristics of Embedded OS

• Direct use of interrupts
• permits user process to use interrupts directly
• no need to go through OS interrupt service routines
• have efficient control over a variety of devices

Developing an Embedded OS

• Two general approaches
• Take an existing OS and adapt it for embedded purposes
• Design a purpose-built OS solely for embedded use

Adapting an Existing OS

• Examples include Windows, Linux
•  familiar interfaces facilitate portability
•  slower and less predictable than special purpose embedded OS
•  not optimized for real-time and embedded applications  require considerable modification to achieve adequate performance
• optimizes for the average case rather than the worst case for scheduling
• assigns resources on demand
• ignores semantic information about an application

• Adapting an Existing OS

• Need to add
• real-time capability
• streamlining operation
• other specialized and necessary functionality for the given device

Purpose-Built Embedded OS

• Most OS are designed from the ground up for embedded applications
• Typical characteristics include:
• Has thread switch (fast and lightweight)
• Real time scheduling policy
• Small size
• Responds to external interrupts quickly (<10 s)
• Minimizes intervals during which interrupts are disabled

• Purpose-Built Embedded OS

• Typical characteristics include:
• Provides fixed or variable sized partitions for memory management and the ability to lock code and data in memory
• Provides special sequential files that can accumulate data at a fast rate

Purpose-Built Embedded OS

• To deal with timing constraints, the kernel:
• provides bounded execution time for primitives
• maintains a real-time clock
• provides for special alarms and timeouts
• supports real-time queuing disciplines, e.g., EDF (earliest deadline first)
• provides primitives to delay processing and to suspend/resume execution